If one wants to feed power that is generated in local d.c. generators, e.g. photovoltaic generators, fuel cells, etc., into an a.c. power system of a power supply company, the d.c. voltage must be converted using a power inverter into an a.c. voltage that conforms to the grid. In this context, the power levels are in the private user range of approximately 1 to 5 kVA.
Typically, for the power range mentioned, there is only a single-phase feed into the grid, but this can lead to unintended and undesired asymmetries of the three-phase system when there is great penetration of small feeders, especially in the local district. Moreover, with single-phase feeding, an intermediate storage of the power is necessary for the inverter, because although the d.c. generator, e.g. photovoltaic generator, supplies constant power, no power may be fed into the grid around the phase zero-crossing of the a.c. current. For example, in Germany, according to the VDEW guideline “Parallel operation with the low-voltage grid”, power levels above 4.6 kVA must generally be fed into the grid of the power provider in three-phase mode. A single-phase feed is not allowed.
An attempt is therefore made, even at lower power levels, to carry out a three-phase power feed. For this purpose, voltage conversions of the type specified at the outset were used, with either three single-phase inverters having been indirect-coupled or a d.c./d.c. controller and a jumper having been linked to a two-step concept. A single-step design, in any case, has the disadvantage that it may only cover a modest input voltage range but would be necessary, for example, in view of the different interconnection variants of photovoltaic modules.
A microprocessor-controlled, single-phase inverter has become known, for example, from German patent 196 42 522 C1, in which five controlled switches and one inductor are used. If these inverters are modified to produce a three-phase version, the number of switches in particular increases substantially, namely to fifteen, and three inductors are also needed.
U.S. Pat. No. 5,053,938 A describes a voltage transformer for supplying power to a three-phase motor that includes a single transformer element for producing a positive voltage. Using a bridge circuit having six switches, positive and negative voltages are then produced for the three-phase output.
U.S. Pat. No. 6,212,085 B1 describes a voltage transformer for supplying power to a consumer device in a three-phase wye connection. The transformer uses two controlled switches for each phase and assumes the presence of two voltage sources, in this case batteries, as well as an inductor in the load branch, that is, a motor or a transformer as a load. The circuit also allows no uninterrupted ground connection between input and output, that is, a continuous PEN conductor. The use of this known transformer must therefore remain limited.
An object of the invention is to create an inverter that transforms a d.c. voltage, which is supplied by, for example, solar power systems, fuel cells, batteries, d.c. machines, etc., into a three-phase a.c. current to feed into three-phase grid. In so doing, the use of controlled switches and inductors should be kept to a minimum; nevertheless, a great input voltage range should be covered, whereby the input voltage may be not only smaller than the output a.c. voltage, but also greater.
This objective is achieved, starting from a voltage conversion circuit of the type specified at the outset, by a first converter section being provided to produce positive output voltage portions and a second converter section being provided to produce negative output voltage portions, and the output of the first converter section being connected to the three-phase outputs via one each of first longitudinal phase circuit breakers and the output of the second converter section via one each of second longitudinal phase circuit breakers.
The invention provides a voltage transformer that does not just satisfactorily achieve the objective or objectives in question, but also offers the possibility of enabling a continuous ground connection between the negative (or positive) pole of the input d.c. voltage and the PEN conductor of the three-phase grid, whereupon no stray currents to ground and no mains frequency (50 Hz) fields are output by the d.c. power source, e.g. the photovoltaic generator.
An advantageous variant of an embodiment of the invention is characterized in that a pole of the d.c. voltage is direct-connected to the PEN conductor of the three-phase output, the series connection of a first transverse switch and a transverse inductor is between one pole of the d.c. current and ground, the junction point of this series connection is connected via one each of the second longitudinal phase circuit breakers to the three-phase outputs, the one pole of the d.c. voltage is also conducted via a longitudinal circuit breaker and, in series therewith, a longitudinal inductor via one each of the first longitudinal phase circuit breakers to the three-phase outputs and via a second transverse switch to ground, and the junction point of the series connection of the longitudinal circuit breaker with the longitudinal inductor is connected to ground via a diode situated in the blocking direction in relation to the input voltage, the longitudinal inductor together with the longitudinal circuit breaker and the second transverse switch and one of the first longitudinal circuit breakers as well as the diode as a step-up/step-down converter forming the first converter section, and the transverse inductor together with the first transverse switch and one of the second longitudinal circuit breakers as a negative phase-sequence converter forming the second converter section. In this variant, a continuous PEN conductor is realized and the number of controlled switches is only nine.
An easily realized microprocessor-based variant provides that the drive circuit has:                an input watchdog that is set up at least to monitor input voltage and/or input current,        a grid watchdog that is set up to monitor voltage and/or current and/or phase angle of the three-phase a.c. voltage,        a hierarchical system manager to which the output signals of the input watchdog and the grid watchdog are fed and which is set up to determine setpoint values pertaining to the voltage values to be generated,        a converter drive to which setpoints for the creation of positive and negative output voltages are fed by the system manager in order to drive the switches of the first and second converter sections, and        a phase drive to which setpoints are supplied by the system manager to switch the first and second longitudinal phase circuit breakers in order to drive these longitudinal circuit breakers.        
It may also be expedient if the output of the first converter section is conducted via a first longitudinal diode in the conducting direction to the first longitudinal phase circuit breakers and/or the output of the second voltage converter section is conducted via a second longitudinal diode in the blocking direction to the second longitudinal phase circuit breakers. The advantage of this design is that in this case the requirements imposed on the hierarchical system manager are less complex.